A general optical receiver in optical communication normally includes a light receiving element such as a photodiode (PD) or an avalanche photodiode (APD) configured to convert incident light into a current, and a trans-impedance amplifier configured to amplify the photocurrent generated by the light receiving element. In the light receiving elements, the upper limit of the photoelectric conversion efficiency of the PD is 100% in terms of quantum efficiency. On the other hand, the APD has a function of accelerating, under a high electric field, photoelectrons generated in the element and thus colliding them against a lattice to ionize, thereby amplifying the carrier. For this reason, in the APD, a plurality of carriers are output in correspondence with one photon. Hence, the APD can obtain a sensitivity more than 100% in terms of quantum conversion efficiency, and is applied to a high-sensitivity optical receiver (non-patent literature 1).
A general structure of the APD is a “vertical illumination structure” in which light enters from the upper surface or the lower surface (substrate side) of the element. In the APD, the light responsivity and the operation speed substantially hold a trade-off relationship. That is, in the vertical illumination structure, to increase the light responsivity, a light absorption layer needs to be thick. However, when the light absorption layer is made thicker, electrons and holes generated in the light absorption layer by light reception need to travel a longer distance, and therefore, the characteristic in a high frequency domain lowers. In the “vertical illumination type”, the trade-off between the light responsivity and the operation speed particularly becomes conspicuous.
For the purpose of relaxing the above-described trade-off, an “optical waveguide type” APD has been proposed (non-patent literature 2). In the optical waveguide type APD, the traveling direction of a light wave in the light absorption layer is perpendicular to the crystal growing direction and the transport direction of the carrier. Since the transport distance of the carrier and the penetration length of the light wave in the light absorption layer are independent in the optical waveguide type APD, the trade-off between the light responsivity and the operation speed observed in the vertical illumination type is much less strict. Such a feature of the optical waveguide type is useful not only in the APD but also in a PD. Hence, the optical waveguide type is used in the PD that requires a high speed/high sensitivity.
In the optical waveguide type light receiving element, optical coupling between the optical waveguide and the light absorption layer needs to be implemented such that signal light propagates through the optical waveguide and then finally enters the light absorption layer. Several methods have been proposed to implement the optical coupling. In, for example, a “butt coupling type”, optical coupling between the optical waveguide and the light absorption layer is implemented by making the optical waveguide and the light absorption layer abut against each other (see non-patent literature 3). In the butt coupling type, a high coupling efficiency can be obtained. However, there is a risk that current concentration occurs due to abrupt light absorption near the optical coupling interface between the light absorption layer and the optical waveguide. On the other hand, there exists an “evanescent coupling type” in which the optical waveguide and the light absorption layer are spatially separated, and the material system between the optical waveguide and the light absorption layer is appropriately designed, thereby implementing optical coupling between the optical waveguide and the light absorption layer using the propagation of an evanescent wave. According to the evanescent coupling type, the concentration of the photocurrent can be relaxed as compared to the butt coupling type.
By the way, to ensure the reliability of the operation in a long term to apply the APD to an actual optical receiver, it is important to inhibit generation of an electric field on an element side surface of the APD (see non-patent literature 4). This is associated with generating a very high electric field inside the element in the APD, unlike a general (conventional) PD. In the general PD, the operating voltage is about 3 V, and the field in the element need only be several ten kV/cm at which the carrier reaches the saturation speed.
On the other hand, in the APD, when ensuring a large operating voltage range and operating the APD with a high gain, an electric field of 2 to 300 kV/cm is invoked in the light absorption layer, and an electric field of 600 kV/cm or more is invoked in the multiplication layer. When such a strong electric field is generated on the element side surface of the APD, lowering of reliability caused by material degradation on the element side surface causes a problem. Hence, in the APD, confining the electric field inside the element is a necessary condition for practical use. For this purpose, an inverted APD or a planar APD has been proposed (see non-patent literatures 5 and 6).